1. Field of the Invention
The present invention relates to a solid-state imaging device, and more specifically to a charge transfer type solid-state imaging device which is capable of increasing the pixel density per bit of a charge transfer portion by k times (k is an integer of 2 or more).
2. Description of the Related Art
Various types of two-dimensional solid-state imaging devices are known. Among them, a charge-coupled device (CCD) type solid-state imaging device has an advantage that it generates less noise. In general, the CCD type solid-state imaging device is classified into two groups: one group relates to an interline transfer system, and the other group relates to a frame transfer system. At the present time, the CCD type solid-state imaging device of the interline transfer system is most commonly used. This is because such a CCD type solid-state imaging device is highly sensitive to short wavelengths due to the use of a photodiode as a photoelectric conversion element and it generates less false signals, called "smears", due to the separately provided light-receiving portion and transfer portion.
FIG. 14A is a schematic plan view showing a structure of a conventional CCD type solid-state imaging device of an interline transfer system; FIGS. 14B through 14K show charge transfer timing. Such a CCD type solid-state imaging device includes a plurality of photosensitive elements (pixels) 1 for converting light into electric charge and accumulating the electric charge. The plurality of pixels 1 are arranged in an array of rows and columns along a first direction (hereinafter, referred to as a vertical direction) and a second direction (hereinafter, referred to as a horizontal direction) which is substantially perpendicular to the first direction. The CCD type solid-state imaging device further includes vertical charge transfer portions 2 each disposed on the adjacent side of each column of the pixels 1 for vertically transferring signals read from the pixels 1, a horizontal charge transfer portion 4 connected to the end of each of the vertical charge transfer portions 2 for horizontally transferring the signals supplied from the vertical charge transfer portions 2, and an output portion 7 disposed at one end of the horizontal charge transfer portion 4 for converting the signals supplied from the horizontal charge transfer portion 4 into image signals to output them to an external device (not shown).
Each of the vertical charge transfer portions 2 has a four-phase structure. Specifically, each of the vertical charge transfer portions 2 is driven by four-phase driving signals .phi..sub.V1, .phi..sub.V2, .phi..sub.V3, and .phi..sub.V4. A pair of pixels which are adjacent to each other in the vertical direction correspond to 1 bit of each of the vertical charge transfer portions 2. For example, pixels P1 and P2, P3 and P4 correspond to 1 bit thereof, respectively.
A CCD charge transfer portion can transfer only one pixel signal per bit. Therefore, as shown in FIGS. 14B through 14K, the conventional CCD type solid-state imaging device effects field accumulation and interlaced reading. Specifically, in a first (odd-number) field, signals output from the pixels P1 and P2, P3 and P4, etc. which are adjacent to each other in the vertical direction are read and added to each other to form one pixel signal. Then, the pixel signal thus obtained is transferred in the vertical direction, followed by the horizontal direction, thereby obtaining an output signal. In a second (even-number) field, signals output from the pixels P2 and P3, P4 and P5, etc. which are adjacent to each other in the vertical direction are read and added to each other to form one pixel signal. Then, the pixel signal thus obtained is transferred in the vertical direction, followed by the horizontal direction, thereby obtaining an output signal.
However, in the above-mentioned operation, a signal for one screen involves 2 fields. Therefore, with a moving subject, its image is likely to be blurred between the fields. Furthermore, since signals output from two pixels are added to each other in the vertical direction, vertical resolution of even a stationary image may be degraded.
As a CCD type solid-state imaging device which is capable of solving the above-mentioned problems, there is a progressive scan type CCD in which all the pixel signals are read in one field without being mixed. FIG. 15A is a schematic plan view showing a structure of a conventional progressive scan type CCD; FIGS. 15B through 15E show charge transfer timing. In FIG. 15A, the components identical with those in FIG. 14A are denoted with the reference numerals identical therewith.
As is apparent from the comparison between FIGS. 14A through 14K and 15A through 15E, one bit of a vertical charge transfer portion corresponds to one pixel in the progressive scan type CCD shown in FIG. 15A, while one bit of the vertical charge transfer portion corresponds to two pixels in the CCD type solid-state imaging device shown in FIG. 14A. In the case of the progressive scan type CCD shown in FIG. 15A, it is required to lead out electrodes of vertical charge transfer portions 2 between the pixels. Therefore, each of the vertical charge transfer portions 2 has a three-phase structure (if it is a four-phase structure, four electrodes are required, resulting in the difficulty in processing) and is driven by three-phase driving signals .phi..sub.V1, .phi..sub.V2, and .phi..sub.V3.
As shown in FIG. 15B, all the pixel signals are read without being mixed in the vertical charge transfer portion 2. Since one bit of the CCD transfer portion corresponds to one pixel, all the pixel signals can be successively read in accordance with an ordinary transfer operation, as shown in FIGS. 15C through 15E. Thus, a progressive scan operation is conducted. Because of this, in the progressive scan type CCD, the image of a moving subject is not blurred, and the vertical resolution of a stationary image is not degraded.
However, in the progressive scan type CCD, the maximum charge handling capacity which the vertical charge transfer portion 2 can handle is limited to the capacity available for one electrode of the three-phase electrodes. More specifically, signal charge can be accumulated only in one electrode of the three electrodes. Therefore, even in the satisfactory case where each electrode becomes equal, the area for charge accumulation of each of the vertical charge transfer portions 2 becomes 1/3 of a pixel pitch in the transfer direction. In the case of the CCD type solid-state imaging device shown in FIG. 14A, a charge amount corresponding to the same length as that of a pixel pitch can be accumulated. As a result, the maximum charge handling capacity handled by the vertical charge transfer portion 2 of the progressive scan type CCD decreases to 1/3 of that of the CCD type solid-state imaging device shown in FIG. 14A.
The decrease in charge handling capacity results in the decrease in the maximum signal amount of an imaging device, i.e., the decrease in dynamic range. When the dynamic range of the imaging device is low, the higher side of the light amount to be handled is narrowed. Thus, a flat image without any depth is obtained, with a bright portion of the image being white. In general, a clear image with a good S/N ratio can be obtained with a brighter scene. However, when the dynamic range is low, a bright scene becomes white; consequently, an image with a good S/N ratio cannot be obtained.
In order to solve the above-mentioned problems involved in the progressive scan type CCD, another CCD type solid-state imaging device is described in A. J. P. Theuwissen et al., "The Accordion Imager: an Ultra High Density Frame Transfer CCD", IEDM Technical Digest, pp. 40-43, December 1984.
FIG. 16A is a schematic plan view showing a structure of a CCD type solid-state imaging device; FIGS. 16B through 16N show charge transfer timing. In FIG. 16A, the components identical with those in FIG. 14A are denoted with the reference numerals identical therewith. The CCD type solid-state imaging device shown in FIG. 16A is different from that shown in FIG. 14A in that each vertical charge transfer portion 2 is driven by a vertical driving circuit 30 on an electrode basis.
The operation of the vertical driving circuit 30 will be described with reference to the charge transfer timing shown in FIGS. 16B through 16N. As shown in FIG. 16B, all the pixel signals are read without being mixed in the vertical charge transfer portion 2. A CCD transfer portion can read only one pixel signal per bit. Therefore, as shown in FIGS. 16B through 16N, electric charge is read successively from a pixel closest to a horizontal charge transfer portion 4, whereby a charge density is decreased to 1/2 and the electric charge is successively replaced for four-phase driving. After being replaced for four-phase driving, ordinary driving becomes possible. Thus, a progressive scan operation in which all the pixel signals are successively read in one field can be conducted. This operation is called an "accordion operation".
However, the CCD type solid-state imaging device shown in FIG. 16A has the following problems.
(1) A signal is read from one pixel, and then, temporarily accumulated in one electrode. Therefore, the maximum signal amount handled by each of the vertical charge transfer portions 2 decrease to 1/2 of that of the CCD type solid-state imaging device shown in FIG. 15A.
(2) A signal read from a pixel portion, which is farther from the horizontal charge transfer portion 4, is stored in the vertical charge transfer portion 2 in a stationary state for a longer period of time. Therefore, a larger amount of noise charge caused by the dark current generated in the vertical charge transfer portion 2 is added to the pixel portion which is farther from the horizontal charge transfer portion 4. The noise charge becomes a fixed pattern noise, remarkably degrading an image quality.